To find the percentage of children who cannot swim, you can use the formula:
\[ \text{Percentage} = \left( \frac{\text{Number of children who cannot swim}}{\text{Total number of children}} \right) \times 100 \]
In this case, there are 6 children who cannot swim out of a total of 30 children. Plugging the numbers into the formula:
\[ \text{Percentage} = \left( \frac{6}{30} \right) \times 100 = 0.2 \times 100 = 20% \]
So, 20% of the children cannot swim.